1. Field of the Invention
The present invention relates to an antenna apparatus provided on a mobile body such as an automobile and a ship for receiving radio waves emitted from a satellite such as a broadcast satellite.
2. Description of the Related Art
In a conventional antenna apparatus for a mobile body, as described in Japanese Patent Unexamined Publication No. 02-159802, a planar antenna is divided into a plurality of antennas and a drive signal in an azimuth direction and an elevation direction of the planar antenna is produced from a phase angle indicative of a delay phase of a received signal of a second antenna with respect to a received signal of a first antenna, and an electric motor is driven through a motor driver on the basis of the drive signal to control an attitude of the antenna and the antenna may automatically track to always face toward the satellite. Further, in order to determine whether the antenna catches the satellite or not, an elevation angle of the antenna is changed at intervals of a predetermined pitch and the antenna is rotated in the horizontal direction at each elevation angle. Thus, when a side lobe peak power of the received signal exceeds a predetermined threshold level S, it is judged that the antenna catches the satellite. Japanese Patent Unexamined Publication Nos. 2-206779 and 2-216074 disclose inventions relating to the present invention.
An automatic gain control (AGC) of a tuner unit of the conventional antenna apparatus is attained by a feedback circuit in order to maintain the received signal at a constant level. When radio waves are cut off, the tuner unit of the antenna apparatus is automatic-gain-controlled to maintain a gain of the tuner unit at the value at which the antenna apparatus has caught the satellite, to thereby make determination of a peak position and detection of a main lobe on the basis of signals received up to that time. Such a conventional method has no problem regarding the peak determination and detection of the main lobe, but since a half-power angle of the main lobe is narrow, it has a problem of a narrow detectable width in scanning made on the basis of a signal obtained by subtracting a threshold level from the received signal. Consequently, it is necessary to make narrow a set interval of elevation angle of the antenna apparatus resulting in many times of scanning, so that a time required to retrack the satellite is increased.